As is known in the forest products industry, the need for more accurate machinery to produce standard size finished lumber products has increased as the cost of the timber resource used in making lumber products has increased in value. Moreover the increased accuracy must be preferably accompanied by increased speeds of overall operations.
Standard planing machines now in use take a fixed planing cut from the lumber with the bottom cutterhead with the remaining material being removed by the top cutterhead. The minimum bottom planing cut must be sufficient to remove sawing irregularities caused by the least accurate sawing machine in the sawmill, requiring the set cut from the lumber by the bottom cutterhead to be significantly greater than the very thin cut that would be required to cleanly plane the bottom of the most accurately sawed lumber from the sawmill. Because it is impractical to separate and separately plane the accurately sawed lumber and inaccurately sawed lumber from the sawmill it is necessary to set the depth of the bottom cutterhead planing cut sufficiently large to remove the most severe sawing irregularities, and thereby requiring that all sawing machines in the sawmill be set to saw lumber thickness with sufficient planing allowance for a heavy, i.e. deep, bottom cutterhead planing cut.